The swing is empty. The roundabout is turning but there is nobody on it. You call out, but there is no reply. You shout louder. Nothing. You scream a name and people turn to stare. They see panic. "Help me!!" you think, but who can you trust? Who has taken your child? You feel sick, helpless, tiny, unable to move.

Hope flares up - she'll run out from behind that bush, chuckling, and you'll sweep her up in your arms, trembling, weeping into her hair with relief - but then the terrors return. She has been stolen. Abducted. She is being murdered or abused somewhere, right now, while you stand here useless. She is crying out for you - "Mummy! Daddy!" - but you can't hear her. You cannot help her. You cannot save her........

Most people are lucky and their child is found quickly. But for others this awful, paralysing moment when hope and fear fight never ends. It just goes on and on. Avril Clarke is trapped in such agonies right now. Her daughter Sasha McLeish went missing in Luton on 23 June. The 15-year-old visited a fair, then said goodbye to her friends at 10pm before starting to walk home. "It has been a horrible nightmare since that day," says Mrs Clarke. "I can't understand why this has happened."

For the past few days a picture of Sasha's face has been posted on the back of 450 white vans operated by the engineering company Emcor, which has done the same in the United States and has seen 28 children found. This is the first scheme of its kind in Britain. In a month's time another face will take Sasha's place. "You hear the stories," says Terry Whale, chief executive of Emcor UK and a father of two, "and you think, 'What if it was my family?'"

All children who will appear on the vans are listed on a police website, and shown above. Among them is the young, smiling face and infant curls of Alan Davidson, who was three years old when he disappeared. He was playing with his little sister Donna and their Afghan hound Kissie in the front garden of their home in Irvine one spring morning. Gran was watching them, but she turned away for a moment. Just a moment. The dog ran out of the gate into the road and Sandy followed. Donna toddled on behind, or so she has been told. The dog and the girl came back, but Sandy did not. He never came back. That was 30 years ago.

"I rack my brains trying to remember some clue to what happened, but I can't," says Donna. "I was so young. It was so traumatic. My mind has gone blank." She has even been hypnotised in an attempt to dredge up a childhood memory of the incident. "I did see a man, and Sandy going away with him. I came round crying, saying, 'No, no, no!'"

A child goes missing every five minutes, according to one estimate. Others suggest that up to 130,000 disappear every year. Some children are just testing the boundaries. Some are on the run from abuse. Some have been thrown out of their homes. Some have been taken by a father, mother or other family member. Some have just got lost. A few have been taken by a stranger. But the really shocking thing is that nobody knows how many.

The estimates are based on samples. They include boys and girls who run away from home for a night or two, maybe two or three times a year. More than 95 per cent of lost children are found or return within two days, says Inspector Ravi Pillai of the Police National Missing Persons Bureau. But when he is asked how many children are missing right now, at this moment, the inspector shakes his head. "I couldn't tell you."

That seems astonishing. If he doesn't know, then who does? "Nobody." Why not? For a start, local police forces have different methods of recording missing persons. They don't talk to each other easily, which is why the bureau exists. The police national computer says about 5,000 people are missing at any one time, but here's the crunch: there is nobody to say how many of them are children. The information is there on the computer. The police can match a name or description to any other individual quickly, as part of an impressive response to a high-risk disappearance that can include helicopters with heat-seeking equipment and news flashes on local television. But nobody is collating and analysing the data to get an overall picture of how many children go missing, where and why.

"We know there is a real need and we are making that case, but it is also very resource intensive," says Inspector Pillai. "Burglary and murder have a higher priority than us."

Helen Southworth, Labour MP for Warrington South, is among a group of MPs trying to do something about that, by introducing new laws or badgering the Treasury for more money. "There are great things going on in local partnerships between the police, children's services and the voluntary sector," she says, "but they are not being co-ordinated nationally."

The Children's Society has just started working with the Department for Education and Skills to assess how to set up a national network of refuges for children who run away. "There is no safety net for people under 16 if they run," says a spokeswoman. "For a long time, people were blissfully unaware that there were children sleeping on the street at all."

Once a child has been missing for a fortnight (or much sooner if the case is high profile) their details are passed to Inspector Pillai's unit at New Scotland Yard. On his wall there is a picture of an American cop in a hamburger joint leaning over to listen to a mop-headed boy, whose stick and handkerchief suggest he has run away from home. The bureau borrowed an American idea to set up www.missingkids.org.uk, which currently shows 69 faces. Some of them are not reproduced above for legal reasons. The results are remarkable: about 75 per cent of the children whose images appear are "recovered" in one way or another. That could mean a happy reunion with their families. Or it could mean a body has been been found.

"The majority are alive, and we are satisfied we do not need to intervene any further," says Inspector Pillai as we walk down the corridor to meet his colleagues. "What do you expect the bureau to look like?" he asks with a smile, prompting thoughts of rows of gleaming computers staffed by busy workers. "Here we are." The reality is six desks in a corner of a poorly-lit, open-plan room overlooking St James's Park station. Pinned to the wall are the pictures of lost children.

Alan Blackburn has been working here since the bureau opened in 1994. He gives an example of the kind of work they do: "If a person goes missing in Yorkshire and a body is washed up in Kent those two forces will have sent us the details separately, and we can make the match." On his desk are framed photographs of his own children. Does it not affect him, dealing daily with heart-rending details of disappearance? "I try not to think about it, but it did sink in when we met Donna Davidson and other relatives earlier this week at the launch of the van scheme, and they were in tears."

Davinia Darch, the team manager, interrupts. "The thing to remember is that less than 3 per cent of these children have been abducted by strangers. It is much rarer than it can seem." Alan agrees. "That is comforting."

But not for Donna, who grew up seeing posters of her brother all over town. Whenever another child disappeared there would be reporters at the door. The habits formed then have not been broken. "My mum and dad still want to know exactly where I am all the time, even at my age," says the 32-year-old. "I'm like that with my own kids."

Her son Brendan is six and "the spitting image of Sandy". History came horribly close to repeating itself at Easter this year when he was being looked after by a neighbour. They live at Saltcoats, on the Ayrshire coast. "My neighbour turned her back on him for five minutes and he was away. He had a great adventure of it with his friends, down on the beach." Brendan was missing for more than four hours. "It was terrible. I was frantic. The only thing I could think of was, 'I can't tell my mum.' I couldn't put her through that again."

Donna has only recently been able to accept that her brother may be dead. "I think he was abducted. I would like to think he has been brought up as their child and had a good life, but the chances of that are slim, I know."

Donna, a natural sceptic, even contacted a medium whom she heard was involved in the search for Holly Wells and Jessica Chapman, and posted him a cowboy suit her brother once wore. "He told me that he believed Sandy had been murdered within a day of going missing, by a man who had been arrested for doing things to children in the past. He told me I was lucky to get away." Did she believe him? Her voice trembles with the hurt and confusion she still feels after all this time, and which is shared by all those who have lost young boys and girls they loved, out of the blue. Hope and fear are still at war. "I sort of believe him. In a way it is comforting. Not much else is."

WHAT HAPPENS WHEN A CHILD DISAPPEARS

3.30pm Sally, a 12-year-old girl, sets off from school to walk home alone. She is usually prompt. When Sally is 15 minutes late her mother starts ringing round friends to ask if they have seen her.

4.30pm The police are called. Officers take a description from Sally's parents and note the circumstances. They must now decide how serious the risk to Sally is. Is there reason to suspect she has been attacked or abducted?

5pm Sally is believed to be in immediate danger. The police set up a command team and deploy all available resources. This may mean dogs and helicopters searching the route from Sally's school to home.

5.30pm A Child Rescue Alert is put out. Sally's photo and description are circulated to radio and television stations which issue a news flash. This is done only in the most serious and urgent cases.

48 hours later Most vanished children have been found by now. Sally's details go to the Police National Missing Persons Bureau in London, which can match them against any body or child found by another force.

14 days later Sally is now re-garded as a long-term missing person; her picture is posted on a website. It may also appear on Emcor's 450 white vans. There is still hope: 75 per cent of cases put online are resolved. Most are found alive.

Contact the Police National Missing Persons Bureau on 0808 100 8777 or go to www.missingkids.org.uk

"One disappears every five minutes. So how many are lost in Britain right now? The scandal is that nobody knows..."